drm/rockchip: inno_hdmi: fix an error code

We were accidentally returning PTR_ERR(NULL) which means success when we
wanted to return a negative error code.

Fixes: 412d4ae6 ('drm/rockchip: hdmi: add Innosilicon HDMI support')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Yakir Yang <ykk@rock-chips.com>